Studies of populations exposed to fallout from nuclear tests or accidents are a major focus of this project. The Chernobyl nuclear reactor accident in Ukraine has led to an unprecedented increase in thyroid cancer among children in Belarus, northwestern Ukraine, and bordering Russian provinces. With the assistance of the Department of Energy (DOE) and the Nuclear Regulatory Commission (NRC), NCI has organized long-term follow-up studies of 39,000 children in Belarus and 34,000 in Ukraine. To date, the thyroid glands of approximately 20,000 subjects have been screened. In a case-control study of thyroid cancer in Belarus, with individual dose estimates, a statistically significant dose-specific link between thyroid cancer and I-131 was demonstrated.Villagers living along the banks of the Techa River in Russia were exposed to chronic external and internal radiation from the Mayak nuclear facility. Cancer mortality is being evaluated in a cohort of approximately 28,000 people who received large doses of radiation from the radioactive waste dumped into the river. Current efforts focus on tracing the original population for vital status information; and on dose reconstruction, using the Joint Coordinating Committee for Radiation Effects Research dose estimates. The Radiation Epidemiology Branch conducted a thyroid screening study of 3,000 residents of heavily exposed and lightly exposed villages near the Semipalatinsk Atomic Bomb Weapons Test Site in Kazakstan exposed to radioactive fallout, who were under 15 years of age at exposure. Screening of thyroid glands by ultrasound and needle biopsy was performed by a team of physicians from the U.S. and Kazakstan. Efforts are currently underway for dose reconstruction and to link the estimated individual doses to questionnaire data and biological markers of dose.